Voice communication has been the dominant application in wireless networks. Cellular standards, such as Global System for Mobile Communication (GSM) and IS-95 were originally intended for voice traffic only. With the recent growth of the Internet, voice and high-speed data are now offered over wireless networks. One issue has been improving the physical layer efficiency of packet data services.
Third generation wide-area wireless networks include the introduction of packet data services over a CDMA network, in particular, the introduction of fast rate adaptation and smart scheduling over CDMA downlinks. More recently, incremental redundancy protocols have been proposed with a view towards gaining “time diversity” for short transmissions. One such protocol, which works well with the channel changes and smart scheduling, is the Adaptive Asynchronous Incremental Redundancy (A2IR) protocol. Versions of this protocol were recently adopted as part of the 1xEVDV as well as high-speed downlink packet access (“HSDPA”) standards.
FIG. 1 is a block diagram illustrating a portion of a prior art wireless communication system. As shown in FIG. 1, the wireless access network typically includes an access terminal (AT) 130, a communication transmission device such as a Base Station Transmission System (BTS) 120, and a communication system control device, such as a Base Station Controller (BSC) 110. The BSC 110 receives data from a core network and passes the data to the BTS 120 so that the BTS 120 can transmit the data to the AT 130 over a wireless link 125. Data is communicated in the reverse direction as well and passed from the AT 130 to the core network. The connection between the BTS 120 and BSC 110 is typically through a Ti link 115. A cluster of BTS units may be connected to a router, which in turn is connected to the controller via a T3 link.
A key to efficient delivery of the data packets transmitted from the BTS 120 to the AT 130, has become how efficient the transmission/re-transmission of data is scheduled and transmitted, given the redundant nature of a protocol, such as A2IR. It should be understood that FIG. 1 is merely exemplary of the networks which may host such transmission/retransmission.
As an introduction to MCS, the concept of a hybrid automatic repeat request (ARQ) protocol is helpful. Considering the forward link of a wireless network, e.g. wireless link 125, and given an information block and some knowledge of the channel conditions, the network may proceed to choose a code rate and modulation, then transmit packets of data accordingly. AT 130, upon reception of the packet, conducts a cyclic redundancy check (after demodulation and decoding) in order to determine the efficiency of the transmission.
In second generation, and early versions of third generation data systems, if the CRC check did not pass, the packet would be deemed in error and it is left to the link layer ARQ, (e.g., the radio link protocol (RLP) and network layer ARQ protocols, such as the transmission control protocol (TCP)) to recover from the errors. In other words, complete re-transmission of physical layer packets was the only way to recover from link errors. However, hybrid ARQ (HARQ) protocols, which work on the physical layer, allow for the reuse of symbols/coded bits that were deemed erroneous, via “soft combining” of frames, with a view towards better utilization of physical layer resources.
For reuse, if after one transmission from BTS 120, an error is detected, the receiver does not discard the bits but instead retains them with the hope of combining them with future transmissions. Each transmission on its own may be erroneous, but the ensemble of transmissions may allow the frame to recover from link errors. Two conventional classes of hybrid ARQ protocols include chase combining (CC) and incremental redundancy (IR). The former protocol allows for combining of modulation symbols, whereas the latter involves the combining after demodulation but prior to decoding.
Referring to A2IR, “adaptive” refers to the channel sensitive nature of the protocol where the modulation and channel coding scheme (MCS) is varied between retransmissions. “Asynchronous” refers to the time asynchrony that is allowed between successive transmissions (in order to work with the underlying scheduling algorithm). This leads to challenges of designing “smart scheduling” algorithms as well as “rate (or modulation and coding scheme (MCS)) selection.”
Two constraints imposed by conventional HARQ protocols operating with other upper layer protocols on physical layer resource allocation are the residual packet error rate (PER) constraint and the maximum number of re-transmissions constraint. In order to ensure that individual user “packet call” delays are kept within limits, a reasonably low error rate should be presented to upper layers. In the interest of controlling average packet call delay, the number of physical layer (HARQ) re-transmissions permitted for a single packet cannot be unbounded. The notion of residual PER makes sense only with a finite number of HARQ re-transmissions. In practical systems the number of such re-transmissions is quite limited (e.g. 4).